120 DOVE ON THE ELECTRICITY OF INDUCTION. 



closed within a tube of sheet iron did not increase its action. 

 If however the iron tube which separates the electro-magnet 

 from the induction-spiral is of very thin metal, and has a consi- 

 derable diameter, then the shocks are very perceptible both when 

 the tube is entire or cut open lengthways*. A solid electro- 

 magnet also, one-half of which is surrounded by an entire gun- 

 barrel, the other half by a gun-barrel cut open lengthways, 

 does not destroy the equilibrium of two spirals which previously 

 compensated each other with respect to the galvanometer, when 

 one was enclosed by the entire and the other by the cut gun- 

 barrel; whence it follows, that in this case the discontinuity of 

 the tubes is not an indispensable condition. But with bundles 

 of wires the following phaenomena are observed. 



29. When an entire iron tube, in its inducing action as re- 

 gards the galvanometer, compensates the action of one that is 

 cut lengthways, this compensation remains almost complete 

 when any number of wires are placed into one or the other tube, 

 i. e. with bundles of wires which are enclosed within entire and 

 cut tubes, the inducing action as measured by the galvanometer 

 is dependent entirely upon the enclosing iron. It is however 

 different as regards the physiological action. In this case the 

 action of the wires enclosed within the tube is nearly destroyed 

 when the suiTounding tube is entire, but not when the tube is 

 cut open. 



30. The results thus obtained for hollow cylinders having the 

 dimensions of gun-barrels are somewhat modified when the 

 tubes ai'e made of sheet iron. As regards the galvanometer, the 

 wires exert an action through them, so that, Mhen wires are 

 placed within one of the cylinders, the galvanometric action of 

 that cylinder is increased. When the welded tubes were in- 

 serted one into the other, and the same was done with the cut 

 tubes, and these latter were so placed towards each other that 

 the sections should correspond, then the action of the wires 

 placed within them was observed to be less than when they were 

 enclosed in entire or cut tubes of simple sheet iron. 



* Upon one of the limbs of an electro-magnet 28 inches in length, and sur- 

 rounded with G5 coils of copper wire 2^ lines in thickness, was placed a 

 coil of wire 4 inches 2 lines wide, 500' long and half a line tliick, and the 

 shocks of the current induced by this coil were tested when connection between 

 the electro-magnet and the galvanic current was broken. Tlie current re- 

 mained almost quite as powerful wlien a cylinder of thin sheet iron 35 lines 

 wide, first welded and then cut open lengthways, was interposed between the 

 electro-magnet and the induction-spiral. 



